Sealing against the passage of fluids--gases or liquids--between faying surfaces is a very old art. "Faying" surfaces are those which directly abut each other, or which almost abut each other with an intermediate body between them such as a sealant. Built-up structures are replete with faying surfaces. The substrates bearing the surfaces are customarily joined together by fasteners such as rivets or threaded combinations whose purpose is to hold them against separation and to limit or prevent relative shear movement between them.
Especially in structures which are subject to a variety of environments such as substantial temperature change, and deflection due to bending or vibration, relative movement and separation between local regions of confronting faying surfaces cannot be completely prevented. Thus even those flat surfaces which are practically attainable when brought together will in use permit leakage of fluid between them.
This is a tolerable situation where the confinement of a fluid is not required. Many faying surfaces are joined without regard for sealing. However in applications such as fuel tanks, leakage of fuel is not tolerable. In aircraft fuselages, leakage of air from a pressurized cabin must at least be minimized. There are numerous other examples in aircraft and spacecraft as well as in ground based structures where fluid containment (gas or liquid) in a built-up structure is required.
This is an old problem, and it has been solved in various ways which utilize sealants applied to appropriate parts of the structure. With rivets or fasteners in areas where sealing is critical, beads of sealant are applied around exposed edges of adjacent surfaces, often in the form of wet sealants which cure or dry after assembly. The techniques for application and subsequent clean-up are both expensive and labor intensive, and complicate the assembly, maintenance and repair of the structure.
In aircraft construction, a wet layer of sealant is often placed between the faying surfaces at the time they are joined together, and the sealant cures after the assembly is completed. The process is designed in such a way that the entire interstitial area will be filled. To assure this, an excess of sealant is applied before the surfaces are brought together. Some of the sealant is expelled from between the surfaces when they are joined, and their excess sealant must be removed from the adjacent area around the sealed edges. The problems created by the excess sealant are not trifling. The surrounding area becomes a mess that has to be cleaned up, and all excess sealant must be carefully removed. If the sealant contains toxic additives such as chromates, the toxic excess requires careful disposal methods which are expensive. The expensive disposal extends to auxiliary items such as the cloth used for removal, brushes used for application, and the like. Worker protection must be provided against contact with such a sealant, requiring the use of masks and gloves. Solvents such as 1,1,1-trichloroethane and ether solvents which are used for the cleanup bring their own hazards.
In order to assure adequate filling of the interstitial region, it is not sufficient merely to provide an excess of sealant. It is also necessary to provide a uniform excess. This requires a further tooling step to rake and trim the exposed wet surface to a uniformly thick area of a configured shape.
These labor intensive procedures are costly. They must be performed a reasonably short time before assembly so the sealant remains fluid while the assembly is completed. This is a serious limitation on the freedom to schedule production, because the surfaces cannot be prepared long before assembly and then wait their turn for use.
The above labor and economic problems and shortcomings of a sealant which is applied wet at the time of assembly are severe. They also involve the economic problems of material waste and structural weight penalties. The waste of expensive sealant material which must be wiped up and disposed of is the lesser of these.
Of far greater consequence is the weight penalty. It has been calculated that on a large aircraft such as the C-17, the use of the dry sealant system of this invention can reduce the total sealant weight required by about 800 pounds compared to the weight of a wet sealant even when the wet sealant is applied in an optimum manner. It should be remembered that weight is an extremely expensive quantity in aircraft and spacecraft, because each pound and structure to support it requires fuel to raise it every time it is lifted. It has been estimated that in aircraft, each pound costs about hundreds of dollars over the useful life of the aircraft.
A sealant which can be applied well before assembly and handled while dry can be made to closer tolerances, without applying excess sealant to assure that there is enough. This potentially avoids the most troublesome and costly problems. In addition, the part can be prepared long before it is needed for assembly, and can be used when it is most convenient to the production schedule.
However, attempts to coat one or both of the surfaces, drying the coating, and then joining them has not previously been successful. The reasons reside in the stringent conditions the sealant layer must fulfill.
To be successful for its intended purpose the sealant must be dry so that it can physically be handled without changing the shape of the sealant layer, or fouling the surroundings. It must not extrude to become a nuisance after assembly, and the sealant must ultimately come into complete conformity with both surfaces. The surface to which it is directly applied will assuredly be fully abutted. However, the exposed other surface of the dry sealant must effectively contact and engage the other surface (or the exposed surface of an opposite sealant layer). Accordingly, in the substantial total thickness required for a practical sealant, often bridging surfaces from between about 0.005 and 0.01 inches apart, down to near contiguity, the dry sealant must be deformable, but not be liable to substantial cold flow. This is assured by control of the physical properties of the cured sealant.
In order to be practical, the thickness of the dry layer must be consistent and readily applied. Surfaces to be covered come in a wide range of sizes and configurations, from long spars and wide panels to intersections with tight corners. While techniques such as spraying, brushing, rolling and flooding can in many situations effectively be used, in general from the point of view of production efficiency, spraying is the preferred method. For this reason sprayable coatings are the most desirable manner of application.
In order to be practical for production purposes the uncured sealant should not contain any solvents or volatile materials (although in some cases the use of water as a solvent might be acceptable). If solvents are present they present ventilation problems, as well as fire and explosion hazards. In addition solvent evaporation from the film can lead to pinholing and film shrinkage with subsequent possible development of leaks.
The sealant must be strongly adherent to its substrate and be compatible with both its substrate, with other sealant compounds which are used to form fillets or beads, and with tackifiers and other adhesives if they are used. Such other compounds are characteristically applied as a backup as reassurance against leakage through the spacing between the faying surfaces, and to resist corrosion when a corrosion resistant substance is provided as an additive.
While a heat curable sealant is useful in this invention, the application of curing temperatures to many substrates could cause warpage or other damage to a substrate such as an aircraft panel. It is preferable for the sealant to cure at room temperature, for example between about 60 degrees F and about 120 degrees F.
A substantial pot life is desirable when a pre-mixed liquid sealant is applied. The term "pot life" is less meaningful if the sealant is a multiple component mixture that is mixed in a dispensing nozzle. In this case cure time after application is a more appropriate term. While cure times can vary from minutes to a week or so, a cure time not much longer than about 16 hours is most practical in a manufacturing venue. An overnight cure of about 16 hours is about as long as a manufacturing operation is likely to tolerate. After the cure is completed, there should be no limitation on how long the sealant may remain exposed. Certainly it should not be so long as to require a large number of parts to be treated in advance and held in inventory while the sealant cures.
The long list of constraints continues with the requirement that the sealant resist corrosion and solvent attack by many common substances. These substances include such frequently-encountered examples as air, water 1,1,1-trichlorethylene, halogenated hydro carbons, aromatic solvents such as toluene, common solvents such as ketones (MEK), esters (butyl acetate), alcohols (methyl and ethyl), and hydrocarbon fuels such as JP8.
Production of a continuous and uniform sealant layer is essential. For example, some polymer systems are very sensitive to the presence of water, which can generate void inclusions. The sealant must be readily mixed in convenient apparatus in a conventional manufacturing environment.
Other requirements, especially for aerospace operations in which the use of this invention will be most frequent is the ability to withstand and operate over a wide range of temperatures, generally between about -65 degrees F and about 250 degrees F or wider. The capacity to be repaired if damaged is essential. Suitability for repair requires that a later-applied application of the sealant can form a continuous bond with a contiguous remaining layer of undamaged sealant.
In view of this array of requirements, which steadily become more demanding as the complexity, size, and ambient and physical conditions become more severe, it is not surprising that the concept of utilizing dry sealant layers to seal between faying structures has been neglected. The use of more expensive, complicated, and labor intensive sealing techniques utilizing wet sealants which are later cured in place have become the accepted mode despite their cost and other disadvantages.
It is another object of this invention to simplify and reduce the cost of a reliable seal between faying surfaces, at the same time providing one which is more reliable and much less likely to require repair.
It is yet another object of this invention to provide structure comprising a pair of assembled substrates with faying surfaces bridged by a cured sealant layer according to this invention.
Sealants of this type are often applied to surfaces that are subject to corrosion by the environments in which they are used. Metal aircraft structures made of materials such as aluminum, titanium and composites are examples. To counter this risk, sealants customarily include a corrosion resistant component. By far the most extensively used substances for this purpose are metal chromates, particularly strontium chromate, zinc chromate, and barium chromate, and their mixtures. These function well for this purpose, and heretofore have enjoyed widespread and usually uncritical acceptance.
However, chromates themselves have become environmentally objectionable. Their handling in manufacturing and disposal operations has become more regulated and troublesome. In some applications the mere presence of chromates per se has become an issue. Still, the protection of surfaces intended to remain in service for many years, even decades, and in which there is very limited access for inspection, and repair is excessively expensive even where it is possible, the use of adequate corrosion resistance components is essential. For this purpose chromates are much to be preferred, their effectiveness for very extending periods of time having been proved long ago.
It is another object of this invention to provide a non-chromate corrosion resistant element for sealants, especially effective for sealants between faying surfaces, but also effective in other types of coatings as well.